An actual conventional “linear” amplifier, such as an RF amplifier, produces some distortion, such as generating second and third harmonics of the fundamental frequency, generating frequency-mixed signals, and generating intermodulation products. For example, a “linear” amplifier receiving sine waves f1 and f2 will output the following signals, having various magnitudes. The second and third order signals are output due to distortion:
TERMOUTPUTFREQUENCYlinearfundamentalf1, f22nd order2nd harmonic2f1, 2f23rd order3rd harmonic3f1, 3f22nd orderfrequency mixing(f2 − f1), (f2 + f1)3rd order3rd order intermod. products(2f2 − f1), (2f1 − f2)
The output current of the amplifier can be described by the following power series, limited to the third order:iout=gm1Vin+gm2Vin2+gm3Vin3,where the expansion coefficients gm1, gm2, gm3 are equal to:gm1(Vin)=dIout/dVin gm2(Vin)=½d2Iout/dV2in=½dgm1/dVin gm3(Vin)=⅙d3Iout/dV3in=⅓dgm2/dVin 
Here, gm1Vin is the linear amplification, gm2Vin2 is the second order signals and mixing products, and gm3Vin3 is the third order signals and third order intermodulation products.
The third order intermodulation (IM3) products are the most problematic since they may occur near a fundamental frequency and may be difficult to filter out. IM3 products can also be generated by the second order interaction (mixing) of the second harmonic and the fundamental frequency. If an inverted second harmonic is generated from the original signal by a predistortion circuit, and the inverted second harmonic is then combined with the original signal through second order mixing, such predistortion can cancel out the IM3 products. Predistortion circuits are common.
A typical predistortion circuit for cancelling IM3 products receives a signal split from the original signal (f), then frequency-doubles it (2f), then adjusts the level of 2f, then phase shifts the frequency doubled signal with a delay circuit to invert the signal, then bandpass filters the resulting signal, then combines the resulting signal with the original signal at the input to the amplifier. In another example, a calculated predistortion signal is combined at the output of the amplifier.
Such a predistortion circuit connected in parallel with the original signal path to the amplifier typically uses circuitry, such as a splitter, that cannot easily be put on an IC chip, much less put on the same IC chip as the amplifier circuitry. Such circuitry may include a transformer. Thus, such predistortion circuits add expense and take up circuit board real estate.
U.S. Pat. No. 6,414,545 describes a single-port predistortion circuit connected to an input node of the amplifier that uses a reversed biased diode junction of a transistor to generate the predistortion signal. The transistor size must be related to the size of the input transistor of the amplifier (e.g., 1/16 the size) to control the amount of predistortion. The reverse bias breakdown voltage of a transistor or diode varies significantly due to process variations. Therefore, the predistortion is difficult to control. Also, the reverse breakdown voltage is much higher than a diode's forward voltage. Therefore, generating a relatively high voltage is required to implement a reverse bias predistortion injector. Other drawbacks of that design also exist.
What is needed is an efficient predistortion circuit that does not have the drawbacks of the prior art predistortion circuits.